The cleanest case for war is self-defense against an army already crossing the border. Two harder cases have dominated recent debate. May a state strike before it is attacked? And may outsiders use force inside another country to stop its government from slaughtering its own people?

Preemption and prevention

The first step is a distinction. Preemption strikes an enemy that is about to attack; prevention strikes to stop a threat that might grow in the future. Preemption resembles the person who fires first at a man raising a gun. Prevention resembles the person who attacks a neighbor because the neighbor is buying guns and might one day use them.

The classic legal test for preemption comes from a nineteenth-century incident. In December 1837, during a rebellion in Canada, British and Canadian forces crossed into New York, seized an American steamboat called the Caroline that had been supplying the rebels, set it on fire and sent it adrift toward Niagara Falls. One American was killed. In the diplomatic quarrel that followed, the US Secretary of State Daniel Webster wrote in 1841 that such action could be justified only by a necessity "instant, overwhelming, leaving no choice of means, and no moment for deliberation." Webster's test allows a first strike only when the necessity is instant, overwhelming and leaves no time to deliberate.

Many thinkers find Webster's test too strict for modern weapons. Walzer proposed a looser one. Walzer argued that a state may strike first when it faces manifest intent to injure, active preparation and real danger in waiting. His main example was June 1967, when Israel struck Egypt's air force after Egypt had ordered UN peacekeepers out of Sinai, closed the Straits of Tiran to Israeli shipping and massed troops near the border. Historians still debate whether Egypt actually meant to attack, which shows how much these judgments depend on reading an enemy's mind.

Prevention has a much worse reputation. Writing in 1625, the Dutch jurist Hugo Grotius called it "quite untenable" that one could take up arms to weaken a growing power that might someday become dangerous. Grotius rejected war merely to weaken a growing power that might one day threaten others. The reasoning is simple: if mere possible danger justified war, every state could attack every neighbor.

Two modern cases test that view. In June 1981 Israeli aircraft destroyed Iraq's Osirak nuclear reactor before it began operating. The UN Security Council condemned the Osirak strike unanimously, with the United States voting in favor, though some officials later praised it. In 2002 the United States set out a strategy claiming the right to act against threats before they were fully formed, and in March 2003 it led an invasion of Iraq, citing weapons of mass destruction. The Iraq Survey Group found no stockpiles of weapons of mass destruction. The lesson critics draw is that prevention rests on predictions that can be badly wrong, and the price of error is paid by others.

Humanitarian intervention

The second hard case reverses the usual picture: the victims are a government's own people. In 1859 John Stuart Mill, in an essay titled A Few Words on Non-Intervention, argued for great caution. Mill argued that freedom won for a people by outsiders seldom lasts, because the habits that sustain liberty grow only through a people's own struggle.

The case against intervention rests on history as much as theory. Opponents warn that strong states can use rescue as a pretext for domination, and many invasions have been launched in the name of protecting someone. The case for intervention replies that sovereignty exists to protect people, and that a government massacring its own citizens has forfeited the shield. The failures to act in Rwanda in 1994 and at Srebrenica in 1995 gave that reply great force, and in 2005 the world's governments endorsed a responsibility to protect populations from mass atrocities.

Worked example: Kosovo, 1999

In 1998 and early 1999, Yugoslav and Serbian forces under Slobodan Milošević fought Albanian insurgents in Kosovo and attacked Albanian villages. In January 1999 some forty-five Albanian civilians were killed at Račak. Talks at Rambouillet in France failed. On 24 March NATO began bombing without Security Council approval, because Russia and China opposed force. The campaign lasted seventy-eight days. During it, Serbian forces drove hundreds of thousands of Kosovo Albanians from their homes. In June Yugoslavia agreed to withdraw and a UN administration took over.

Apply the tests. The cause was the protection of a persecuted population, a strong humanitarian claim. Authority was the weak point: no UN approval. An international commission later judged the campaign unlawful but legitimate, a phrase that captures the tension exactly. Last resort was argued both ways; critics said the terms offered at Rambouillet were ones no sovereign government would accept. Conduct raised questions too. NATO aircraft flew high to keep pilots safe, which critics said shifted risk onto civilians on the ground; Human Rights Watch counted roughly five hundred civilian deaths from the bombing. Supporters reply that the expulsions were reversed and most refugees returned home within weeks.